The possibility of mirror-matter is something taken seriously by some physicists, and some of what is printed on the site in your link looks valid; I have to say though, that there's some stuff in your link that looks fishy, though (i.e. the whole deal about the Tunguska explosion - just as easily explainable by an ordinary comet impact). Always remember, the web is an incredibly vast repository of information with zero quality control; always check the sources of info before taking it too seriously.
In any case, mirror matter is an interesting concept - it may well be real (though once again, I'm not an expert on this).
Yes it is, and I first heard about it approximately 2 years ago. The gentleman who's site I included seem to be one with the most knowledge and I believe he has articles in a few journals. I know however that there are many crackpot sites and theories floating about on the Internet.
What intrigues me most is that if true, a mirror world might exist with all the implications that would mean.